Numerous applications, such as solid-state electricity metering and electricity grid automation devices, require accommodation of high voltage AC as input power yet must provide low voltage/high current DC output power for use by analog and digital circuitry. The power available in these environments, known as “line power,” is typically supplied by an AC electric power utility and is usually within the range of 80 VAC and 600 VAC. The line power is the only power available for use with these types of applications, and the circuit board area and enclosure volume available to accommodate the power supply is often very limited.
Conventional systems attempt to provide AC to DC conversion, as presented in detail for example in U.S. Pat. No. 6,169,391, in four broad categories of power supplies: the transformer approach, the high voltage linear regulator approach, the high voltage capacitive coupling approach, and the switching power supply approach.
The transformer-based power supplies approach uses a step down transformer and some type of wave rectification. However, the disadvantage to all transformer approaches is the large size, cost, and power consumption of step down transformers, or the large size of other components such as capacitors that are used in conjunction with smaller transformers.
The high voltage linear regulator approach eliminates the large, costly step down transformer, but has the disadvantage of large capacitors and high power dissipation requirements.
The high voltage capacitive coupling power supplies approach also eliminates the step down transformer and reduces power consumption but adds design complexity and requires large capacitive elements.
The switching power supplies approach produces low voltage DC from high voltage AC by switching at a high frequency such that transformer size can be reduced. However, the transformer and switch elements in switching power supplies must be rated high enough to withstand the line voltage and switching transients. The filter capacitors at the input to switching power supplies must be rated to withstand the maximum line voltage and are required to have enough capacitance to maintain the voltage ripple within acceptable limits at the minimum line voltage. These two conditions result in physically large capacitors. These high voltage elements greatly increase the size and cost of switching power supplies and make it difficult to use these power supplies in space constrained applications, such as solid-state electricity metering and electricity grid automation devices.
For example, FIG. 1 is a diagram of a conventional switching power supply used to convert the AC line voltage 110 and produce DC output voltage 170. The power supply includes a bridge rectifier 120 and a DC-DC converter 100. It will be understood by those skilled in the art that the filter capacitor 130, the switch 140, and the transformer 150 all must be rated to withstand the peak of the maximum input voltage 110 with an adequate margin of safety. For example, for 600VAC input this peak voltage is 848.5V. Thus, the filter capacitor 130, the switch 140, and the transformer 150 must be capable of withstanding 848.5V plus any switching transients that may be generated. The qualitative relationship between maximum input voltage (X-axis) and the size of the switching power supply (Y axis) is shown in FIG. 2. Exponential growth curve 200 indicates the relative effect of accommodating a large maximum input voltage on power supply size.
Some switching power supplies are commercially available as single chip solutions with an external switch. For example, a company called Supertex Inc., based in Sunnyvale, Calif. (see http://www.supertex.com) currently manufactures gating integrated circuits (ICs), such as the SR086 and SR087, which implement gating functions in a small SO-8 footprint. One of Supertex's patents, U.S. Pat. No. 6,169,391, discloses a device shown schematically herein in FIG. 3, which rectifies and regulates high voltage alternating current without the use of transformers, large capacitive coupling circuits, or high voltage linear regulators. The device includes a rectifier 320, a control circuit 330 for sensing the output voltage 350 of the rectifier 320 and switching on and off the input power, a storage capacitor 380 and a low voltage linear regulator 340. The control circuit 330 effectively divides the device into a high voltage subsystem 310 and a low voltage subsystem 315. Although this device allows conversion of high voltage AC to low voltage DC without the use of transformers, large capacitive coupling circuits, or high voltage linear regulators, the available current at the output 370 is less than 100 mA, which is not sufficient or suitable for use by solid-state electricity metering and electricity grid automation devices or any other application/components that requires more power or current.
Further, power supplies based on this type of design have typically attempted to produce logic level voltages (e.g., 3.3 V, 5.0 V) by reducing the gating-on time to a very low value. This results in very short duration high amplitude current spikes being drawn from the AC line, which, in turn, causes noise issues and also limits the available current to less than 100 mA, which reduces output power. Efficiency is also reduced because at small conduction angles, the time required by the switch to transition between the ‘on’ state and the ‘off’ state is a significant percentage of the total ‘on’ time. This transition period is a highly dissipative state of the switch and causes losses due to heating.
FIG. 4A through FIG. 4D illustrate a voltage waveform at different points in the circuit of FIG. 3. As shown in FIG. 4A, the voltage waveform 400 of the input 350 to the control circuit 330 is a rectified form of the input voltage at the same magnitude as the input voltage. The typical output from control circuit 330 for such an input 350 would be the voltage waveform 410 as shown in FIG. 4B, in which the circuit is closed whenever the full wave rectified voltage is below a prescribed threshold voltage 440, such as 40 Volts. However, the waveform 420 in FIG. 4C shows how the output 360 of the control circuit 330 is altered due to the presence of capacitor 380 in the circuit design of FIG. 3. The low voltage linear regulator 340 of FIG. 3 then produces the regulated DC output voltage waveform 430 as shown in FIG. 4D, though at a limited output power as noted above.
There is therefore a need for improved systems, devices, and circuit designs for converting high voltage AC to low voltage DC without the use of large high voltage filter capacitors or large high voltage switching power supplies, while also providing for high current DC outputs.
There is a further need to provide methods, systems, circuit designs, and devices to reduce the size and cost of a power supply module.
There are additional needs to provide methods, systems, circuit designs, and devices to increase the input voltage range of a DC-DC converter of a given size.
There are additional needs to provide methods, systems and designs to increase the input voltage range of a low voltage switching power supply of a given size.
There are further needs to provide methods, systems and designs to be able to use a low voltage (less than 80VDC input voltage range) DC-DC converter in high voltage (80 to 600V) applications.
There are additional needs for methods, systems and designs, wherein high voltage AC is not allowed to propagate beyond a full wave rectifier and a transistor switch.
There are additional needs for methods, systems and designs, wherein a filter capacitor is required to be rated to only withstand a low voltage DC and not high AC line voltage.
There are yet further needs for methods, systems and designs, wherein the output power of a power supply does not change significantly with the output voltage.
There are additional needs for methods, systems and designs, wherein the need for large capacitive circuits and high voltage switching power supply is eliminated.